Pentax is probably the digital camera company most ready to do a larger-than-FF DSLR, since they've been doing relatively cheap and full-featured larger-than-FF camera systems for decades. This isn't super cheap, but it certainly isn't exotically expensive, seems to be very well built, and the lenses are cheap for what they are. The 70.87mm flange distance is also short for the format, so any old MF lenses except Mamiya can be adapted (at least in theory)... no hope of using most 35mm lenses with a glassless adapter, but very few would cover the sensor well anyway. It's also worth noting that the longish flange distance is good for sensor angle of incidence.

The Sony 50MP sensor is great, but 4.4x3.3 isn't exactly 6x4.5 (the size implied by 645) nor even the 5.6x4.15 that were the true image dimensions on film for the 645. One could argue that odd crop factors have been pretty much the norm in medium-format digitals anyway.

Overall, this doesn't excite me, but it should sell very well... probably with most of the profit going to Sony's sensor sales... and encouraging the development of more big sensors.